1. Field of the Invention
The present invention relates to a method of producing a surface acoustic wave device such as a surface acoustic wave filter or other device, and more particularly, to a method of producing a surface acoustic wave device having bumps formed by a flip chip process.
2. Description of the Related Art
In recent years, mobile communications equipment has been greatly reduced in size and weight. Also, it has been required to reduce the sizes of electronic parts for use in the mobile communications equipment. Thus, for surface acoustic wave devices used as band filters in mobile communication devices, miniaturization has been urgently required.
In conventional surface acoustic wave devices, surface acoustic wave elements are contained in a package. Electrode pads of the surface acoustic wave elements are electrically connected to electrode patterns provided for the package via wires. Accordingly, miniaturization of the surface acoustic wave devices has had a limit.
In recent years, the flip chip process has been utilized more frequently in which bumps are formed on electrode pads on a surface acoustic wave element, and the bumps are arranged to contact an electrode pattern on a package and are bonded to the pattern, e.g., by application of supersonic waves.
Japanese Unexamined Patent Application Publication No. 9-172341 discloses an example of a method of producing a surface acoustic wave device, using the above-mentioned flip chip process. In the surface acoustic wave device described as the conventional technique, a surface acoustic wave element is covered with a dielectric layer in order to enhance the anti-dust property and the corrosion resistance.
In particular, first, interdigital electrodes and planar electrodes defining electrode pads are formed on a piezoelectric substrate. Then, a dielectric layer defining a protection layer is formed on the piezoelectric substrate. Moreover, for electrical connection to the external upper electrodes each having the same areas as those of the planar electrodes when seen in plan view, are arranged such that the upper electrodes overlap the planar electrodes via the dielectric layer. Moreover, by bonding metal bumps to the upper electrodes, the dielectric layer is broken by the upper electrodes, so that the planar electrodes and the upper electrodes are electrically connected directly to each other. The metal bumps, after they are connected to the planer electrodes as described above, are connected to a package substrate in the above-mentioned flip chip process.
On the other hand, Japanese Unexamined Patent Application Publication No. 10-163789 discloses the structure in which surface acoustic wave filter elements can be mounted onto a circuit substrate without the element being contained in a package. In particular, as shown in FIG. 8, a surface acoustic wave element 52 is mounted directly to a circuit substrate 51. For the purpose of realizing the above-mentioned mounting, the surface acoustic wave element 52 is formed as follows.
That is, first, interdigital electrodes, a reflector, an input electrode pad, and an output electrode are formed on a piezoelectric substrate. Then, a protection film 53 made of SiO2 and having a film thickness of 200 xc3x85 to 15 xcexcm is formed. Furthermore, the central portions of input-output electrode pads 52a are exposed from the protection film 53. An electro-conductive bonding agent 54 is made to adhere thereto. As the conductive bonding agent 54, a solder ball, a gold bump, or an electro-conductive resin is used. The conductive bonding agent 54 is arranged to contact a corresponding electrode pattern 51a of the circuit substrate 51. In the state in which the surface acoustic wave element 52 is placed onto a circuit substrate, the conductive bonding agent such as the solder ball is heated and melted, whereby the surface acoustic wave filter element is mounted directly to the circuit substrate 51.
Japanese Unexamined Patent Publication No. 4-371009 discloses a problem that a surface acoustic wave device likely experiences a pyroelectric breakdown due to a potential difference between electrodes which is caused by deposition of a passivation film on a piezoelectric substrate and heat applied during the formation of a window in the passivation film. To this end, the Publication discloses a method to connect between an input terminal and an output terminal via a connection wire before formation of a passivation film and disconnect the connection wire after formation of the passivation film, whereby the short-circuit between the input and output terminal prevents the pyroelectric breakdown.
In the case in which a surface acoustic wave element is bonded to a package substrate or a circuit substrate by use of a bump, it is necessary to heat a piezoelectric substrate during the process of forming the bump in order to promote the interdiffusion of metal, so that the bump shear strength can be increased.
However, in the case of a piezoelectric substrate having pyroelectric properties used, a potential difference is generated between the electrodes of the interdigital transducers as the surface acoustic wave element, due to changes in the temperature caused by the above heating is carried out. Thus, discharge occurs. Problematically, this discharge causes the interdigital transducer to be pyroelectrically broken, so that the acceptance ratio is reduced.
In addition, it is impossible to measure frequency characteristics of a device before formation of a passivation film if the input and output terminals are short-circuited before formation of the passivation film to prevent a pyroelectric breakdown. As a result, it is impossible to adjust an operation frequency of a device by changing a thickness of a passivation film. Even if it is possible to adjust an operation frequency of a device by disconnection a connection wire after formation of a passivation film, measuring an operation frequency of a device and changing a thickness of a passivation film, thereafter, there sill remains a problem of a pyroelectric breakdown as the connection wire has been cut during a formation of bumps.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of producing a surface acoustic wave device which can be mounted onto a package substrate or a circuit substrate, using a flip chip process, in which pyroelectric breaking of the interdigital transducer, caused by heating during formation of a bump, is prevented, and variations in characteristics are prevented, and the acceptance ratio is greatly improved.
According to a preferred embodiment of the present invention, a method of producing a surface acoustic wave device by use of a flip chip process, includes the steps of forming on a piezoelectric substrate, at least one interdigital transducer and a plurality of electrode pads electrically connected to the interdigital transducer, forming bumps on the respective electrode pads and providing an insulating film at a region other than a region where the bumps are formed.
In the present preferred embodiment, the insulating film greatly improves the dust resistance and erosion resistance in a surface acoustic wave device. Further, it is possible to miniaturize an electronic apparatus such as a communication apparatus in which a surface acoustic wave device is used as a surface acoustic wave device can be mounted on a circuit board by a flip-chip bonding method using the bumps.
The method may also include the step of forming an insulating film on an entire surface of the piezoelectric substrate, and removing a portion of the insulating film at least on the bumps.
In the case, since the portions of the insulating film on the bumps are removed, it is possible to easily mount the surface acoustic wave device on a circuit board by a flip-chip bonding method.
The method may further includes the step of etching a surface of the electrodes after the step of forming an insulating film.
In the case, the residue at the interface between the electrode pads and the bumps is removed. Therefore, it is possible to increase the strength of bonding between the electrode pads and the bumps. Further, since the bump area is enlarged due to the press of bumps during the bonding by ultrasonic energy application, the enlarged portion of the bump may further improve the bonding.
The step of forming the at least one interdigital transducer and electrode pads may include the step of forming a short-circuit wiring for short-circuiting between a signal terminal side and a ground terminal side of the at least one interdigital transducer, the method further including a step of cutting the short-circuit wiring between the second and third steps of the method.
In this preferred embodiment, the short-circuit wiring for short-circuiting can prevent a discharge by a pyroelectric effect. As a result, destruction of electrode fingers in the interdigital transducer is eliminated, which prevents deviation in the characteristics of the surface acoustic wave devices.
Also, it is possible to measure the frequency characteristics before forming an insulating film. Thus, it is possible to surely provide a surface acoustic wave device which has a predetermined characteristics. Further, the pyroelectric breakdown at the formation of bumps is also prevented.
The method may also further include, between the first and second steps, the steps of forming a conductive film on the back surface of the piezoelectric substrate which is opposite to the side where the interdigital transducer is formed, and short-circuiting the electrodes on the front surface of the piezoelectric substrate to the conductive film on the back surface by use of a conductive film or a conductive jig.
These steps prevent charges from accumulating and discharging at the surface of the piezoelectric substrate. Thus, it is possible to omit a short-circuit wiring and reduce the number of the manufacturing steps.
In the first step of the method, a plurality of interdigital transducers and a plurality of electrode pads electrically connected to the interdigital transducer may be formed on the piezoelectric substrate so as to form a filter circuit, and a conductive pattern substantially surrounding the filter circuit may be formed on the piezoelectric substrate.
Such a conductive pattern helps the charge at the front surface of the piezoelectric substrate towards the back surface of the piezoelectric substrate, thereby further enhancing the charge transfer.
The bumps may be made of the same material as that of metal pads which receive the bumps so as to improve the inter diffusion between the electrode pads and the bumps.
The thickness of the insulating film formed in the third step of the method may be selected to adjust a frequency of the surface acoustic wave device. In this case, the manufacturing steps may be reduced.
Alternatively, the third step of the method may include the steps of forming an insulating film on an entire surface of the piezoelectric substrate, and reducing a thickness of the insulating film so as to adjust a frequency of the surface acoustic wave device.
Further, the method according to preferred embodiments of the present invention may include the step of measuring the frequency of the surface acoustic wave device, before the step of adjusting the frequency of the surface acoustic wave device.
In this case, it is possible to realize high accurate frequency adjustment, thereby increasing production yield.
Other elements, features, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown.